Wireless communication systems are known to include a plurality of wireless communication devices that communicate directly (e.g., point-to-point) or through an infrastructure. For direct communications, a wireless communication device, such as a radio, cellular telephone, station coupled to a personal computer or laptop, et cetera, transmits data on a particular radio frequency channel directly to another wireless communication device. For infrastructure-supported communications, a wireless communication device transmits data on an assigned radio frequency channel to an access point (or a base station). The access point determines the targeted wireless communication device from the received RF signals. If the targeted wireless communication device is affiliated with the access point, the access point transmits the data to the targeted wireless communication device on a radio frequency channel. If the targeted wireless communication device is not affiliated with the access point, the access point forwards the data to a central station, which routes the data to the access point that is affiliated with the targeted wireless communication device.
To ensure reliability of data transmissions within a wireless communication system and to ensure interoperability of differing manufacturers' equipment, standards have been developed. Such wireless communications standards include IEEE8 02.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution services (LMDS), multi-channel, multi-point distribution systems (MMDS), and/or variations thereof.
Such standards prescribe operating parameters for particular types of wireless communication systems. For example, the IEEE 802.11a standard defines a wireless local area network that prescribes a frequency band of use, division of the frequency band into channels and sub-channels, encoding/decoding convention, modulation/demodulation convention, frame format, data rates, et cetera. Further, the IEEE 802.11a standard provides various combinations of data rates and modulation schemes, which can be selected via a coding rate corresponding to a particular modulation scheme.
Within any standard compliant wireless communication system, a wireless communication device transmits and receives data via RF (radio frequency) communication paths, i.e. RF channels. To transmit data, a wireless communication device encodes and/or modulates the data in accordance with the applicable standard to produce encoded data. The wireless communication device then mixes the encoded data with a local oscillation in one or more stages to produce an RF signal. The wireless communication device transmits the RF signal via an antenna.
To receive data, a wireless communication device receives an RF signal via an antenna, filters the RF signal and amplifies it. The wireless communication device then mixes the filtered and amplified RF signal with a local oscillation in one or more stages to produce a baseband signal. In an ideal environment, the baseband signal will be identical to the encoded data produced by the wireless communication device that transmitted the RF signal. Since wireless communication devices do not operate in an ideal environment, the baseband signal produced by the receiver is not identical to the encoded data transmitted by the transmitter due to the transfer function characteristics of the RF channel and of the receiver. In particular, the baseband signal may be phase, amplitude, and/or frequency offset from the encoded signal.
To compensate for the phase, amplitude, and/or frequency offset, most receivers include frequency equalization (FEQ). One method of frequency equalization is to force frequency domain errors to zero. While this is the simplest approach, it forces some frequency components to have zero error that do not need correction, thus, in some instances, the zero forcing over compensates the baseband signal. Other methods include linear prediction and adaptive frequency equalization. Linear prediction calculates the frequency response of the RF channel to determine the appropriate response for frequency equalization. While more accurate than zero forcing, it is very complex thus expensive in cost and in integrated circuit real estate. Adaptive FEQ adjusts the frequency equalization at the data rate. For IEEE802.11a applications, adaptive frequency equalization is impractical for moderate to low cost solutions due to its complexity and speed requirements.
Therefore, a need exists for a method and apparatus that enables wireless communication devices to adequately and cost effectively compensate for the frequency transfer functions of the RF channel, the transmitter, and/or the receiver that adversely filters received signals.